Automatic detector gain control circuit



Sept. 21, 1937. H. E. HOLLMANN 2,093,561

AUTOMATIC DETECTOR GAIN CONTROL CIRCUIT Filed Dec. 5, 1954 INVENTOR f/Al/J 5/9/06 HOZM/MW ATTORNEY Patented Sept. 21, 1937 UNITED STATES AUTOMATIC DETECTOR GAIN CoNTRoL CIRCUIT Hans Erich Hollmann, Berlin, Germany, assignor to Telefunken Ge sellschaft fiir Drahtlose Telegraphic In. b. H., Berlin, Germany, a corporation of Germany Application December 5, 1934, Serial No. 756,029

In Germany January 27, 1934 Claims.

The invention relates to a receiver circuit in which an automatic volume control takes place in dependence upon the intensity of the receiving carrier wave.

5 In order to eliminate fluctuations in the volume caused by atmospheric phenomena of fading, and to adjust the reception of transmitters whose arriving waves are of different intensities to an equal volume at the place of reception, the

30c carrier wave.

general procedure has been to derive a direct voltage from a detector which is proportional to the mean high frequency amplitude. This voltage has been applied to the control electrode of one or more high frequency amplifier stages as bias in suitable polarization. In this manner the amplification attained in the controlled stages is varied in accordance with the mean receiving amplitude in such a manner that the high frequency voltage applied to the detector has either an entirely constant, or approximately constant, value. This type of control involves special requirements of the properties of the characteristics of the controlled tube if a sufficient range of control is to be obtained, and if distortionsare t0 be avoided.

In accordance with the present invention the control is carried out in the detector proper in that the sensitivity of the detection is influenced in dependence upon the mean amplitude of the This is suitably accomplished in such a manner that to a grid electrode inserted in the discharge path used for the detection ,a'

direct voltage is applied furnished by the detector path, and proportional to the voltageof the carrier wave. In this way the preliminary amplifier stages can be continuously kept adjusted to an optimum of the amplifying action, and is no longer necessary to use special tubes in these stages.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims, the invention itself, however, as toboth its organization and.

I into efiect.

In the drawing:

Figs. 1 and 2 show two circuit embodiments of the invention.

Fig. 1 shows a self-controlling, retardingaudion circuit in which a tube V is used with, for instance, an indirectly heated cathode K, aretarding electrode B and two grid electrodes. The grid A next to the retarding electrode B receives a high positive bias, while the retarding electrode B has a low positive, or negative, bias applied to it in the known manner. The modulated high frequency oscillations supplied by the oscillatory receiver circuit E are brought into action between the cathode K and retarding electrode B,'and they are detected upon the nonlinear portion of the retarding characteristic ib=f(eb). ib, eb designating the retarding current and retarding voltage respectively. At the same time the condenser C will be charged to the mean value of the detected alternating voltage E0.

The detectedcurrents produced in the retarding electrode circuit are transmitted to the grid anode A through current distribution, and are taken therefrom by means of an output transformer T.

The retarding electrode B is suitably connected to the grid anode A by means of a condenser Cu having a capacity of a few hundred cm., whereby it is accomplished that the high frequency potential acts at the same time upon the retarding electrode B and positive grid anode A. Since the characteristics of these two electrodes, in view of the current distribution taking place therebetween, are such that a decrease of the current of the one electrode corresponds to an increase of the current of the other electrode, the high frequency current variations annul each other entirely, or to a large extent, so that the load of the input circuit furnishing the control potential will be diminished. Thechoke D prevents the passage of the high frequency into the grid circuit. The working point on the retarding characteristic is determined by the voltage drop produced at the resistor W, and the bias Ew tapped from the voltage divider P which bridges the source of grid potential.

In order to carry out the volume controlin accordance with the invention, the direct potential produced at the condenser C is applied to the control grid G, disposed between cathode K and grid anode A. The desired working point can be adjusted by means of an additional bias Eg. As soon as the input circuit E furnishes a high frequency potential .the following takes place. Due to the direct current a voltage drop occurs through'resistor W; as a result the working point will be changed along the resistance line conforming with the resistance W and extending through the field of the retarding characteristic. At the same time the voltage'on the condenser C increases, and consequently the bias of the control grid G will be displaced in the directionof impressed upon the output transformer T will be reduced, but the voltage at condenser C likewise decreases until a state of equilibrium is reached.

By suitably dimensioning the various circuit elements, in particular C and W, it can be accomplished that the audio frequency voltage be maintained constant independent of the amplitude of the carrier wave supplied to the input circuit E, provided that the input amplitude exceeds a minimum value determined by the threshold value of the arrangement.

It is obvious that the control voltage produced at the condenser C may at the same time also be used for controlling preliminary, or subsequent, amplifier stages in a manner known as such, whereby a considerable spreading out of the range of control can be accomplished. In order to properly carry out the described method of control, the grid anode A should exert but a possibly small reaction upon the electron current which leaves the cathode. Hence it is necessary that the grid anode have a throughgrip through the control grid which is as low as possible. This condition can be attained in that at least one screen grid is disposed between the control grid and the grid anode.

A further improvement will be obtained by the use of a tube having four grid electrodes, a so-called-Hexode. In explaining it is to be understood that the same reference characters are used as in Fig. 1 wherever identical parts are under consideration. The tube V contains the electrodes K, G, A, and B taken over from the previous example.

Between the grid anode A and the control grid G two further grid electrodes are placed; the electrode 'SG next to the control grid G, and for which the term suction grid shall be used, has constant positive potential applied to it while the screen grid '3 next to the grid anode A is maintained at the cathode potential, or at one slightly different therefrom.

The discharge path extending between the cathode K and the retarding electrode B can be considered divided into two sections, designated by R1 and R2 in Fig. 2. The part R1, then, simply represents an electron source for the part R2, and which varies depending upon the potential of the control grid G. The part R2 embodies the retarding audion in the proper sense; the screen electrode S acts as electron emitting surface, hence it plays so to say the role of a cathode. In view of the chosen electrode arrangement a reaction of the potentials produced at the grid anode upon the emission performance will be completely suppressed.

While I have indicated and described several systems for carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention, as set forth in the appended claims.

What is claimed is:

1. In a signal receiving system, a tube provided with a cathode, and at least two cold electrodes, a signal input circuit connected between the cathode and one cold electrode, means for maintaining the latter at a positive potential with respect to the cathode, an output circuit connected between the cathode and the second cold electrode, the latter being at a positive potential with respect to the cathode, and at a positive potential with respect to the one cold electrode, and means responsive to variations in the received signal amplitude for controlling the voltage amplitude in said output circuit, and means for maintaining the two cold electrodes at the same alternating current potential.

2. In a signal receiving system, a tube provided with a cathode, and at least two cold electrodes, a signal input circuit connected between the oathode and one cold electrode, means for maintaining the latter at a positive potential with respect to the cathode, an output circuit connected between the cathode and the second cold electrode, the latter being at a positive potential with respect to the cathode, and at a positive potential with respect to the one cold electrode, and means responsive to variations in the received signal amplitude for controlling the voltage amplitude in said output circuit, a condenser connected between the cathode and the low alternating potential side of the signal input circuit, and a control electrode, disposed in the electron stream from the cathode, connected to the signal input circuit side of the condenser.

3. In a signal receiving system, a tube provided with a cathode, and at least two cold electrodes, a signal input circuit connected between the cathode and one cold electrode, means for maintaining the latter at a positive potential with respect to the cathode, an output circuit connected between the cathode and the second cold electrode, the latter being at a positive potential with respect to the cathode, and at a positive potential with respect to the one cold electrode, and means responsive to variations in the received signal amplitude for controlling the voltage amplitude in said output circuit, said last means including a control electrode, disposed in the electron stream of said tube, which is connected to a point of signal-controlled direct current potential in said input circuit.

4. In a signal detector circuit of a type comprising a tube having a cathode, plate and grid, a signal input circuit connected between the plate and cathode, and an output circuit connected be tween the grid and cathode, means for maintaining the plate at a negative potential with respect to the grid, and said plate and grid being at the same alternating current potential; the improvement which comprises means arranged to control the electron flow in the tube, and said last means being connected to said input circuit so as to vary said flow with the variation in amplitude of detected signal current.

5. In a signal detector circuit of a type comprising a tube having a cathode, plate and grid, a signal input circuit connected between the plate and cathode, and an output circuit connected between the grid and cathode, means for maintaining the plate at a negative potential with respect to the grid, and said plate and grid being at the same alternating current potential; the improvement which comprises means arranged to control the electron flow in the tube, and said last means being connected to said input circuit so as to vary said flow with the variation in amplitude of detected signal current, means providing a negative potential field in said tube between said grid and cathode, and a positive potential field between said last field and the cathode.

HANS ERICI-I I-IOLLMANN. 

